Nemanja Vidic has spoken of his surprise about Chelsea's mid-season slump but also expressed frustration that Manchester United have been too "inconsistent" to build a greater lead at the top of the Premier League.

Sir Alex Ferguson's team have moved six points clear of the champions, with a game in hand, but have struggled at times to find their usual fluency, with Vidic admitting there is considerable room for improvement going into their match at home to Stoke City tomorrow night.

"We must play better," the United captain said. "We believe we can win the league, and we have certainly shown this season we can be solid, but this year the season is very strange. If you'd told me five weeks ago that Chelsea wouldn't win for five or six games ... everyone thought Chelsea were the strongest team and that they would win the title but then suddenly they didn't win.

"That shows how strong and competitive it is in the Premier League. And look how many points we have lost this season. If someone had said we would lose this many points I would have said we would not win the league, that's for sure. But here we are, top and still unbeaten, and if we had won just half of the away games we have drawn we would have been out of reach by now."

United, who have arranged a loan that will send Federico Macheda to Sampdoria for the rest of the season, are unbeaten in their first 19 league matches. They are level on points with Manchester City but have played two fewer matches, and also have a game in hand on Arsenal, who are two points behind. It is a strong position from which to go for a record 19th title but Vidic is frustrated by their erratic performances and a record of two wins and seven draws from nine games on their travels.

"Our away record is not something we are proud of but we have to keep going and hope we can change it. It is hard to explain – definitely in some games we did not perform well but got the result but, then again, sometimes we did play well and didn't get the result. It is very inconsistent from us."

At home United are a much more formidable side, with 28 points out of 30, and have won the previous season's two home fixtures with Stoke 5-0 and 4-0.

Tony Pulis's side, with Robert Huth back from suspension, will be relieved if Wayne Rooney misses out with the ankle injury he sustained in the 2-1 win at West Bromwich on Saturday, but they have not won at Old Trafford since 1976 and have lost 10 of their past 13 games against United without scoring.

"Normally we start getting strong at this time of the season," Vidic added. "That's why we are looking forward to playing Stoke. The only disappointing result [at Old Trafford] was the 2-2 draw against West Brom in October, and we have won all our other home games. It is our fortress, so hopefully we can win the Stoke game too and keep going. Our performances at home have been really good and we may have some injured players back, which helps."

Edwin van der Sar is likely to return for United after missing the win at The Hawthorns because of a virus. Nani also hopes to be involved after a recent hip problem and if Rooney's famed powers of recovery fail him, Javier Hernández or Michael Owen are on standby to replace him.

Ferguson regards Pulis among his close associates in management and he could not resist a little dig at Arsène Wenger as he prepared to face one of the league's more physical sides.

"People have criticised Stoke's style of play but they stick to what they do best. I have great admiration for Tony and the wry way he rides the criticism levelled at his team. If anything the criticism galvanises the players and gives them extra motivation.

"I suppose Arsène Wenger has been one of their main critics but then he does go off on a tangent at times. For instance, he criticised our pitch after they lost here before Christmas and I was at a loss to understand why."

As for Macheda, the 19-year-old has had several offers from English clubs, including Everton and West Bromwich, but he favoured a move to Serie A and will replace Antonio Cassano, who hads moved to Milan. His agent, Giovanni Bia, said the Sampdoria move would be a six-month arrangement. "Then it will be evaluated with Manchester United and they will need to see if he stays in Italy for another year. It is 99% he would go straight back to Manchester."